BOSTON, MA - APRIL 15: A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during the 117th Boston Marathon (Photo by Alex Trautwig/Getty Images)

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Members of the Federal Bureau of Investigation (FBI) stand near the scene of yesterday's bombing attack at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. (Photo by Spencer Platt/Getty Images)

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A Boston police officer stands near the scene of a twin bombing at the Boston Marathon, on April 16, 2013 in Boston, Massachusetts. (Photo by Spencer Platt/Getty Images)

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BOSTON - When runners raced toward the finish line in Monday's Boston Marathon, thousands of fans thought they were simply enjoying a glorious American tradition.

They probably didn't realize that most of them were packing "evidence collection" kits.

Combined, those individual kits, those individual pieces, will make up a puzzle that may ultimately root out the bomber.

While law-enforcement agencies have been actively using social media to generate leads and tips, more are turning to "crowdsourcing" to help them solve crimes.

A term coined in 2006 by Jeff Howe in Wired magazine, crowdsourcing was originally used in business applications -- to raise funds for a startup company or to solve a particular problem by parceling out the task to many. The sum is greater than the parts, in essence.

In Boston on Tuesday, the FBI turned to crowdsourcing, asking the public for any photos and videos that people might have of the marathon scene.

"Assistance from the public remains critical in establishing a timeline of events," said Richard DesLauriers, special agent in charge of Boston's FBI office, speaking at a news conference.

"Somewhere in the background is a guy walking by with a backpack," said Grant Fredericks, an instructor at the FBI National Academy in Quantico, Va. "It could have been someone hours before the bombs went off."

In legal circles, crowdsourcing is gaining momentum.

"There are a lot of people sitting at home with evidence sitting on their hip and they don't know they have it. The goal for law enforcement is to figure out how to collect it," said Fredericks.

He imagines there were probably 200,000 witnesses at the marathon, 70 percent of them with cellphones. And as many as 100,000 people actually took videos and photos, he estimates. Of that, there could be 20,000 to 30,000 hours of video to pore through, if police could get it all, he said. And that's the key.

Because when he trains senior law-enforcement leaders from agencies across the country, he tells them to ask the public for all the raw video they can get. He tells them to not leave it up to civilians, which can "self-censor" their footage.

"I want what may seem to one person to be dull, or the mundane," he said. "But when you put it together, with what others have, you might find the bomber."

Fredericks said being able to organize all the public's contributions is crucial.

He also serves as a project manager at the Law Enforcement & Emergency Services Video Association lab in Indianapolis. LEVA has put together a groundbreaking way for law-enforcement agencies to pool resources to sift through what can be a mountain of video evidence.

When LEVA is "activated," it solicits help from law-enforcement agencies from around the world. Those agencies volunteer their own video experts to comb through a portion of the evidence. The work can be done more quickly and the combined result can be searched, much like an Internet search.

It happened in June 2011 in Vancouver, British Columbia, when Canuck fans rioted after the final game of the National Hockey League's Stanley Cup Playoffs. Rioters left a $3.4 million trail of damaged businesses and vehicles. They also left an electronic path that led prosecutors to file 592 criminal charges against more than 200 rioters.

Vancouver police, determined to pursue the scofflaws, reached out to the public for cellphone video. But they ran into a problem. They received more than 5,000 hours of evidence in more than 100 different formats. They turned to LEVA for help. The lab converted them into one standard, searchable format that let investigators tag events and suspects, leading to arrests.

LEVA has not yet been activated for the Boston bombings, but remains on standby, Fredericks said.

When he coined the term "crowdsourcing," Howe said he could envision that social media could be used to help communities heal. He sees that "benign self-healing" in the public's outpouring of digital help.

The public becomes "like its own organism," said Howe, now a professor of digital journalism at Northeastern University in Boston.

"In the past five or six years, since the advent of smartphones, the rise of Facebook, Twitter, there has developed a much larger cohort of people who instinctively record the news and share it," said Lee Rainie, director of the Pew Research Center's Internet & American Life Project in Washington, D.C., a nonprofit, nonpartisan group that studies the social impact of the Internet.

"People want to bear witness to news they observe or that moves them," he said. Now, more people "think networking first, sharing first."

"It's a powerful and commonplace way for people to participate in their environment," he said.

Picture-sharing is the main currency of social interaction, Rainie said, and public spaces are "celebrity spaces for everyone."

It's also a new tool that law enforcement can exploit, he said. "You never can tell what a photo can show, even at the margins."

Besides law enforcement, other professionals increasingly turn to social media for help in doing their jobs.

Disease researchers have used Facebook, Twitter and Google searches to help track, predict and diagnose illnesses. They have used common descriptions of symptoms, searches for medical information or treatments, or friend connections to study illnesses ranging from the flu to Legionnaire's disease to foodborne-illness outbreaks.

Recently, the federal Centers for Disease Control and Prevention in Atlanta gave a grant to Harvard researchers to see if they could use Google to track dengue fever. Many other experiments are under way.

And, of course, the media increasingly turn to social networking for help in reporting stories. The Guardian in London announced Tuesday the launch of a new digital platform, GuardianWitness, which will allow readers to contribute to live news and other content.

As the bombing investigation continues, the importance of the public's role will become clearer. But, "no matter what agency you're talking about, the public is key," said Darrell DeBusk, a spokesman for the Knoxville, Tenn., police department.

"We cannot have a trooper on the ground everywhere at all times. The public input and the public being vigilant, if they see something that doesn't quite look right, notifying law enforcement -- even if they think it's something trivial -- it could turn out to be something pretty major."